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NBME 16 Answers

nbme16/Block 2/Question#24 (9.2 difficulty score)
An 85-year-old woman is diagnosed with a ...
Morphine is metabolized to active metabolites that accumulate🔍

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submitted by hungrybox(770),

Morphine stimulates mu opioid receptors to provide the desired effect of analgesia, but in doing so can also precipitate many undesired effects.  This patient has multiple signs of opioid toxicity, including miosis (ie, pinpoint pupils), respiratory depression (evidenced by slow respiratory rate and respiratory acidosis), and CNS depression (eg, somnolence, coma).  Morphine is primarily metabolized by the liver via glucuronidation to form 2 major metabolites.  These metabolites, morphine-3-glucoronide and morphine-6-glucoronide, then undergo renal elimination via excretion in the urine.  Because the metabolites are metabolically active, renal dysfunction can lead to metabolite accumulation and opioid toxicity.  Morphine-6-glucoronide is particularly responsible for toxicity, acting as a more potent mu opioid receptor agonist than morphine itself.

Due to its metabolically active and renally cleared metabolites, morphine requires careful monitoring when used in patients with renal dysfunction.  When opioid pain control is needed in such patients, fentanyl or hydromorphone is often preferred as these drugs are predominantly hepatically cleared.

Source: UW18563

Also, morphine-6-glucuronide is excreted renally. So, morphine would cause toxicity in patients with reduced renal functions.

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cyrus_em  @privatejoker Morphine 6-glucuronide is an active metabolite of morphine +  
cyrus_em  Data from clinical studies suggest that M6G contributes significantly to the analgesic effect observed after long-term morphine administration. 2–4 Animal studies show profound and long-term antinociception and respiratory depression after M6G injections (especially when given centrally) through its action at the μ-opioid receptor +1